CONDUIT FLOW: PATHWAYS TO POOR ASR RECOVERY AND SURFACEWATER CONTAMINATION IN FLORIDA

Construction of 330 ‘aquifer storage and recovery’ (‘ASR’) wells is a key component of the approximately $8 billion Congressional appropriation for south Florida’s Everglades Restoration Plan (ERP). Injected ‘recharge’ fluids can include untreated stormwater runoff containing toxic agricultural and urban contaminants, as well as treated effluent. Similar wells in Florida (20 to hundreds of meters deep) are used for aquifer-injection of minimally-treated sewage effluent, stormwater runoff, and agricultural wastewater. Sewage effluent is injected into the aquifer via approximately 1,000 shallow wells throughout the Florida Keys.

‘Recovery’ efficiency (at 250 mg/L chloride concentration) reported by USGS was 3.1%, 2.7%, and 7.2% (‘storage’ periods: 0, 8, and 5 days, respectively) for Lake Okeechobee ‘ASR’ wells proposed for use in the northern vicinity of the ERP. Actual ‘recovery’ efficiency was lower, since chloride concentrations of injected ‘recovery’ fluids were 150, 100, and 70 mg/L, respectively, for those tests. Dye tracer tests conducted this year by USGS in the Biscayne (surficial) aquifer, south of Lake Okeechobee, revealed groundwater travel times of several hours over distances predicted to require days. Evidence that injected fluids resurface in nearshore waters (induced discharge) is seen in stable nitrogen isotope data from macroalgae smothering reefs in southeast Florida.

Those results reflect conduit flow of injected fluids that can cause large-scale subsurface discharge of nutrients and other contaminants into Florida’s sensitive surface waters, including the surf zone. Poor ‘recovery’, rapid travel times, and discharge points for the karst aquifer system are not addressed by groundwater models used for decision-making. Similar flow responses have been addressed in other areas of the regional Floridan aquifer system using a finite-element groundwater model. This approach can identify probable discrete flow paths and discharge points.